1. Field of the Invention
This invention relates to a method and apparatus for compressing image signals. This invention particularly relates to a method and apparatus for compressing image signals, wherein radiation image signals, which represent radiation images of an object, and an energy subtraction image signal, which is obtained from the radiation image signals of the object, are compressed.
2. Description of the Related Art
Techniques for photoelectrically reading out a recorded radiation image in order to obtain an image signal, performing appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems wherein recording sheets, such as X-ray film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often performed on image signals detected from a plurality of radiation images of an object, which radiation images have been recorded on the recording sheets.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which components represent the image information recorded at corresponding sampling points in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
As one of the subtraction processing techniques described above, an energy subtraction processing technique has heretofore been known. In the energy subtraction processing technique, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy distributions. Specifically, by the utilization of the characteristics described above, a plurality of radiation images of an object, in which different images of a specific structure of the object are embedded, are formed with several kinds of radiation having different energy distributions. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure of the object. More specifically, as the plurality of the radiation images described above, a low energy image, which has been formed with radiation containing a comparatively large quantity of low energy components, and a high energy image, which has been formed with radiation containing a comparatively large quantity of high energy components, may be obtained. In cases where the subtraction processing is performed on the low energy image and the high energy image, for example, a soft tissue image, in which only a pattern of a soft tissue of the object is illustrated, or a bone image, in which only a pattern of a bone of the object is illustrated, is obtained as an energy subtraction image in which only a specific structure of the object is illustrated. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037, and Japanese Unexamined Patent Publication No. 3(1991)-285475.
Image signals, which represent the radiation images, such as the low energy image and the high energy image, and an image signal, which represents the energy subtraction image, such as the soft tissue image or the bone image, are transferred to an external device, such as a display device or a printing device. Also, the image signals are transferred to storage media, such as MO, and stored on the storage media. Heretofore, such that the efficiency with which the transfer or the storage is performed may be enhanced, compression processing has been performed on all of the image signals. In such cases, the degree of signal compression has heretofore been set at a value such that the image quality of the image represented by a decompressed image signal, which will later be obtained from decompression processing performed on the compressed image signal, does not become markedly bad. Specifically, of the images described above, the low energy image is the image utilized also in making an ordinary diagnosis, or the like. Therefore, the low energy image signal representing the low energy image has heretofore been compressed with a reversible compressing process, which has the characteristics such that the image quality of the image represented by the decompressed image signal obtained from the decompression processing performed on the compressed image signal, does not become bad, or with an irreversible compressing process utilizing a low compressibility such that the image quality of the image obtained from the decompression processing is capable of being kept to be approximately identical with the original image quality. Also, the high energy image signal representing the high energy image and the energy subtraction image signal representing the energy subtraction image have heretofore been compressed in the same manner as that for the low energy image signal.
However, each of the image signals described above is composed of a very large quantity of data. Therefore, in cases where the signal compression with the reversible compressing process or the irreversible compressing process utilizing the low compressibility is performed, the processing efficiency and the storage efficiency at the time of the transfer, the storage, and the like, cannot be kept sufficiently high.